The invention is in the field of power supplies connected in parallel to supply DC power to a common load. One nonlimiting example is in supplying a large computer requiring high current (e.g., thousands of amperes) at low voltage (e.g., 1.5 to 6 volts). Typically, power supplies of this type now use paralleled switching supplies each having a rated capacity of up to several hundred amperes. Such switching supplies typically draw AC power, rectify and filter it to unregulated DC power, convert the unregulated DC power to a high frequency pulse width modulated waveform (e.g., at 20 KHz), rectify this waveform to provide pulses whose duty cycle is controlled in accordance with the desired DC output voltage, and then filter the pulsed power to provide the desired output DC power. For example, AC power is rectified by a conventional rectifier bridge circuit, filtered by a conventional capacitor filter (or an LC filter) to produce unregulated DC power, and a switching transistor (or transistor network) converts this DC power to a high frequency pulse modulated waveform (i.e., a waveform having a positive pulse followed by a "zero"-volt interval, followed by a negative pulse, followed by another "zero"-volt interval, etc.). This waveform is applied to the primary of a transformer, and the signal at the secondary is rectified, to produce a unipolar pulse train with a duty cycle which is a function of the on and off times of the switching transistor(s). The pulsed power is filtered (e.g., by a conventional LC filter) to provide the desired DC output voltage.
Contemporary switching power supplies typically are most efficient at the 1,000 to 1,500 watt output range. Transistor current capabilities tend to limit standard converter topologies to the area of 1500 W. Therefore, when more DC power is needed, typically several such supplies are connected in parallel, so as to avoid the considerable difficulties attendant in attempting to design a single high output supply. For example, if a computer manufacturer requires 5 volts at 1,000 amperes, typically four power supplies rated at 5 volts and 300 amperes each would be connected in parallel. Ideally, each would supply 250 amperes at 5 volts to the common load, but in practice this is not the case because the respective output voltages, output impedances and temperature coefficients vary slightly but significantly enough as between the different power supplies to make equal sharing unlikely. The typical result is that the power supply which happens to have the highest output voltage at the time would deliver the total current it can until its output voltage is reduced by the output current limiting circuit typically used in such circumstances. Then the power supply with the next highest output voltage would deliver all the current it can until its current limit circuit also would reduce its output voltage, allowing another power supply to become active. As such current limit circuits typically limit at 110% of rated load (or 330 amperes in this example) the result with four paralleled power supplies is that at any one time it is likely three of them would be operating at approximately 330 amperes and a fourth would deliver very little or no current. Of course, it is known that when a power supply operates at its absolute maximum capability, its reliability is significantly reduced. Adding more power supplies in such a parallel system does not in itself solve the problem because under those conditions an extra power supply also is likely to operate either at maximum load or at little or no load.
There have been numerous attempts to remedy this problem. For example, circuits have been proposed which sense the output current of each power supply and send an error signal to the supplies (generally through the sense leads of the individual power supplies) to maintain current sharing. However, this introduces additional circuitry which must be designed for each application, as it is affected by factors such as the number of the paralleled supplies, temperature coefficients and the nature of the power cables from the individual power supplies to the common load, and thus becomes complex, unwieldly and difficult to stabilize, particularly as the number of individual power supplies increases. Attempts have also been made to use a current-driven control circuit which is capable of being paralleled to other supplies in such a manner as to seek to force sharing. However, it is then difficult to achieve stabilization as the number of power supply increases, and it is difficult to implement this approach when it is necessary that the paralleled power supplies provide a choice of output voltages. Examples of attempts to parallel power supplies are discussed in U.S. Pat. Nos. 3,521,150; 4,194,147; 4,359,679 and 4,371,919, the disclosures of which are hereby incorporated by reference.
In contrast to the attempts referred to above, a system and a process in accordance with the invention seek to avoid these and other paralleling problems without having to resort to external circuitry, and with providing the capability of working at selectable output voltages. In accordance with the invention, two or more power supplies are designed and built to be as identical as possible under the circumstances with regard to output voltage and output impedance over the required load and temperature ranges, and their output voltages are controlled in a way causing them to share the load as equally as possible. More particularly, a number of power supplies are paralleled to supply DC power to a common load, and each supply includes an internal circuit which monitors that supply's output current and controls that supply's output voltage to make it decrease linearly at a selected slope from a selected initial nominal level with increase of that supply's output current over a selected current range. The individual power supplies are matched to each other so that the initial nominal output voltage levels of the supplies are substantially identical to each other, and so are the slopes at which the output voltages drop with output currents. The power supplies include provisions for making the operation of the circuits which monitor and control the output voltages and currents, substantially independent of the characteristics (e.g., resistance) of the power cable connecting the power supplies' output terminals to the load. The power supplies embodying the invention connect to the load only through the conventional power cable and sense leads, and do not require any other, special connection.